It is known to use chromatic confocal techniques in optical height or distance or range sensors. As described in U.S. Pat. No. 7,876,456 (the '456 patent), which is hereby incorporated herein by reference in its entirety, an optical element having axial chromatic aberration, also referred to as axial or longitudinal chromatic dispersion, may be used to focus a broadband light source such that the axial distance to the focus varies with the wavelength. Thus, only one wavelength will be precisely focused on a surface, and the surface height or distance relative to the focusing element determines which wavelength is best focused. Upon reflection from the surface, the light is refocused onto a small detector aperture, such as a pinhole or the end of an optical fiber. Upon reflection from the surface and passing back through the optical system to the in/out fiber, only the wavelength that is well-focused on the surface is well-focused on the aperture. All of the other wavelengths are poorly focused on the aperture, and so will not couple much power into the fiber. Therefore, for the light returned through the fiber, the signal level will be greatest for the wavelength corresponding to the surface height or distance to the surface. A spectrometer-type detector measures the signal level for each wavelength in order to determine the surface height.
The '456 patent further describes that certain manufacturers refer to practical and compact systems that operate as described above, and that are suitable for chromatic confocal ranging in an industrial setting as chromatic point sensors (CPS) or chromatic line sensors or the like. A compact chromatically-dispersive optical assembly that is used with such systems that measure the distance to a surface point is referred to as an “optical pen” or a “pen.” The optical pen is connected through an optical fiber to an electronic portion of the chromatic point sensor which transmits light through the fiber to be output from the optical pen and which provides a spectrometer that detects and analyzes the returned light. The returned light forms a wavelength-dispersed intensity profile received by the spectrometer's detector array. Pixel data corresponding to the wavelength-dispersed intensity profile is analyzed to determine the “dominant wavelength position coordinate” as indicated by a peak or centroid of the intensity profile, and the resulting pixel coordinate of the peak or centroid is used with a lookup table to determine the distance to the surface. This pixel coordinate may be determined with sub pixel resolution, and may be referred to as the “distance-indicating coordinate.”
The '456 patent further describes that in normal operation the CPS spectrometer ordinarily receives a certain range or peak region of wavelengths for a certain measurement distance. It is disclosed that the CPS spectrometer may distort the shape of the peak region of wavelengths, and thus influence the corresponding peak or centroid and the resulting distance-indicating coordinate. The systems and methods of the '456 patent provide component calibration data, also referred to as compensation data, that encompasses the effects of wavelength-dependent variations (e.g., non-uniform response) in the CPS spectrometer, and/or the CPS broadband light source. The compensation data of the '456 patent is used to reduce or eliminate errors associated with these effects in the spectrometer and light source. The compensation data of the '456 patent may be redetermined and/or replaced at various points in time, such that the compensation data remains effective to reduce or eliminate errors despite changes in the spectrometer and/or light source characteristics (e.g., due to component aging, environmental variations, or the like).
U.S. Patent Pre-Grant Publication No. 2010/0188742, which is hereby incorporated herein by reference in its entirety, discloses a chromatic range sensor which is a “line sensor” that uses a slit aperture and focuses light along a line rather than a point, providing the capability to measure a distance to a surface at a number of points along that line.
In various chromatic range sensor systems, there is nothing to prevent the detection of more than one spectral peak based on the light reflected from an intended measurement location. In some cases, this is advantageous for measuring the thickness of a transparent thin film. That is, a first spectral peak may correspond to a first distance to the top surface of a transparent film, and a second spectral peak may correspond to a second distance to the bottom surface of that film and/or the surface of the substrate that carries it. However, in other cases, two spectral peaks may occur unpredictably (e.g., due to an unexpected secondary reflection that passes back through a primary or intended measurement location). This may lead to an unexpected and/or erroneous measurement result (e.g., measurement errors that are a significant portion of the measurement range, for example). Providing an improved chromatic range sensor system including a means to identify the presence of measurement errors due to unexpected secondary reflections would be desirable.